Add support for listing claimed messages (transport)

This patch adds a query parameter, "include_claimed", to
the GET messages operation, to allow the caller to specify
whether it wants to include claimed messages in the
listing (as opposed to the default of only listing active
messages).

Change-Id: Ie0ee4a435420fbc3c94a339836853a49b4196460
Fixes: bug #1202430
Merge "Add support for listing claimed messages (transport)"